1. Field of the Invention
The present invention relates to a reduced pressure drying apparatus and a reduced pressure drying method for drying a substrate having the surface coated with a coating liquid for forming a coating film, such as a resist film or an insulating film, under a reduced pressure atmosphere.
2. Description of the Related Art
As one of technologies for forming a resist film by coating the surface of a substrate with resist used in the photolithography, followed by drying the coated resist, known is a method of coating the substrate surface with the resist with a single stroke by using, for example, a coating liquid nozzle having a fine liquid discharging hole. In general, a solvent having a low volatility such as a thinner having a high boiling point is used for dissolving the components of the resist used in such a coating method. Therefore, the substrate is subjected to a drying treatment under a reduced pressure after the step of coating the substrate surface with the resist solution so as to dry the resist in a short time.
FIG. 1 schematically shows the method of coating with a single stroke the surface of a substrate such as a semiconductor wafer with resist. Specifically, a coating liquid such as a resist solution is supplied onto the surface of a wafer W that is held by a substrate holding section (not shown). The wafer W is held in a manner to assume a horizontal posture. Also, for supplying the coating liquid, a coating liquid nozzle 1 having a fine liquid discharging hole and positioned apart from the wafer W in a manner to face the wafer W is reciprocated in one direction (i.e., in the X-direction in the drawing). On the other hand, the wafer W is intermittently moved by a moving mechanism (not shown) in a direction (Y-direction) perpendicular to the reciprocating direction of the coating liquid nozzle 1 in accordance with the timing of the movement of the coating liquid nozzle from one side to the other side of the wafer W. By repeating the particular operation, the entire surface of the wafer W from one edge to the other edge of the wafer W in the Y-direction is coated with a single stroke with the resist solution. Incidentally, a reference numeral 11 shown in FIG. 1 denotes a liquid-receiving mask for preventing the region other than a prescribed region that is to be coated with the resist from being coated with the resist solution.
As a reduced pressure drying unit for subjecting the wafer W coated with a resist solution with a single stroke to the drying treatment under a reduced pressure, known is a unit comprising a hermetic vessel including a cover movable in the vertical direction and a wafer stage. An exhaust port is formed in the central portion of the ceiling of the cover of the hermetic vessel. The exhaust port is connected to a vacuum pump via a pipe so as to make it possible to reduce the pressure inside the hermetic vessel, thereby setting up a prescribed reduced pressure inside the hermetic vessel. A hermetic vessel having an exhaust port formed in the center of the ceiling thereof is disclosed in, for example, JP 2002-313709-A. A measure for preventing the gaseous atmosphere within the hermetic vessel from being locally discharged to the outside is proposed in this patent document. Specifically, it is proposed to arrange a gas flow resistor formed of a porous material above the wafer W in a manner to face the surface of the wafer W so as to form a uniform gas flow over the entire region of the surface of the wafer W.
In the reduced pressure drying unit of the type noted above, the wafer W coated with a resist solution with a single stroke is disposed on the wafer stage and the cover is closed so as to form the hermetic vessel. Also, under the state that the gas flow resistor is arranged at a prescribed height in a manner to face the wafer W, the wafer W is controlled at a prescribed temperature by a temperature control means arranged inside the wafer stage, and the vacuum pump connected to the hermetic vessel is operated so as to set up a reduced pressure inside the hermetic vessel. If the pressure inside the hermetic vessel is reduced to a level close to the vapor pressure of the solvent contained in the coating liquid, the solvent is vigorously evaporated. As a result, a resist film is formed on the surface of the wafer W by the solid component, which is not evaporated, of the coating liquid.
However, since the surface of the wafer W is consecutively coated with the coating liquid as shown in, for example, FIG. 1, a time lag is generated between the starting point and the ending point of the coating. As a result, it is possible for the solvent to be evaporated partially from the coating liquid supplied onto the wafer surface in the initial coating stage before the final coating stage is reached. It follows that a nonuniformity is brought about on the surface of the wafer W in the concentration of the solid component contained in the coating liquid, i.e., the concentration of the resist film-forming component dissolved in the solvent. In other words, it is possible to give rise to a concentration gradient that the solvent concentration is high on the finishing side of the coating and is gradually lowered toward the starting side of the coating. If the drying under the reduced pressure is carried out under the state that the particular gradient is formed in the solvent concentration, the resist in the region having a low solvent concentration is dried earlier. As a result, the solvent flows together with a small amount of the solid component from the region having a high solvent concentration toward the region having a low solvent concentration. It follows that the amount of the solid component is increased on the starting side of the coating operation so as to increase the thickness of the resist film on the starting side of the coating operation. Such being the situation, a resist PR is formed such that the thickness of the formed resist film is increased from the ending side toward the starting side of the coating operation as shown in, for example, FIG. 2.